1. Field of the Invention
The present invention relates to the field of digital-to-analog conversion and, more particularly, to a technique for providing coarse and fine charging of the output filter capacitor of a digital-to-analog converter in order to provide a more continuous analog output signal.
2. Background of the Related Art
The general technique of providing analog-to-digital (A/D) or digital-to-analog (D/A) conversion of signals is well known in the art. Generally, the sampling rate required to sample an analog signal for A/D conversion must be twice the highest frequency component being sampled. This rate is known as the Nyquist rate. More recently, oversampling methods have been utilized for A/D and D/A conversion. In an oversampling type of converter, the sampling rate is much higher than the Nyquist rate. An oversampling technique is described in a reference titled "Oversampling Methods for A/D and D/A Conversion;" by James C. Candy and Gabor C. Temes; IEEE; pp. 1-25; 1992; and "A use of Double Integration in Sigma Delta Modulation;" by James C. Candy, IEEE Trans. Commun., vol. COM-33, pp. 249-258, March 1985.
An advantage of using the oversampling technique is in the precision provided by the converter. With converters operating under the Nyquist rate for sampling, a certain amount of precision is required for the conversion. For example, in converting an analog signal into a 16-bit digital format, 16-bit precision is required. Accordingly, circuits will need to be designed having components which will meet this precision. In many instances, closely trimmed circuit components or precision matching (or compensating) circuits are required to meet the precision.
However, when sampling at a rate much higher than the required Nyquist sampling rate, the oversampling technique permits circuit precision to be relaxed significantly. For example, if the above A/D 16-bit oversampling converter implements an oversampling modulator, the modulator output can be a single bit output. The circuit precision needs only to meet this 1-bit output. Accordingly, closely trimmed circuit components are generally not needed. Additionally, 1-bit precision can be readily met by current generation CMOS (complementary-metal-oxide-semiconductor) components. This fact also applies to digital-to-analog conversion using an oversampling converter.
The disadvantage of using the oversampling technique is the added requirement that the output now needs to be reduced to the standard Nyquist rate at the eventual output of the converter. That is, the higher sampling rate now needs to be returned to the Nyquist rate. In the above 16-bit A/D example, a multiple number of the 1-bit outputs will need to be combined to form a single 16-bit output, which output is equivalent to the 16-bit output from the Nyquist rate converter. With a D/A converter, such a 16-bit discrete signal is converted to a continuous analog signal. However, the oversampling technique is preferred in many applications, since the cost savings in using less precise circuit components outweigh the additional digital signal processing needed at the back end of the converter.
One well known type of oversampling A/D and D/A conversion uses a modulator commonly referred to as a delta-sigma modulator (See the Candy et al. references noted above). Furthermore, the delta-sigma modulator can be configured as a higher-order modulator having multiple integrator stages. See "A Stereo 16-Bit Delta-Sigma A/D Converter for Digital Audio;" by D. R. Welland et al.; Journal of the Audio Engineering Society, vol. 37, pp. 476-486; June 1989; and "A Higher Order Topology for Interpolative Modulators for Oversampling A/D Converters;" by Kirk C.-H. Chao et al.; IEEE Trans. Circuits and Sys., vol. CAS-37, pp. 309-318; March 1990.
In a D/A converter using a delta-sigma modulator, a digital signal is received for conversion to an analog signal. The digital delta-sigma modulator is clocked at the oversampling rate to generate a bit stream which equates to the oversampled rate. Generally, the output is a bit stream of single bits. Subsequently, this bit stream is processed by one or more D/A converter stages to convert the digital bit stream into an analog signal. Then, typically, the signal is filtered by a low-pass filter to remove the noise.
Converters using delta-sigma modulators typically use switched capacitors coupled to operational amplifiers (op amps) to process the signal. Where the switched capacitor circuits convert analog signals to digital signals or operate to process the digital signals through the various circuit stages, the discrete nature of the signal allows considerable latitude in the settling property of the circuit. For example, non-linear settling responses, signal overshoots, signal jitter and other transients are generally not a concern, since the op amps are switched to a non-processing phase during this period. Thus, these distortion introducing responses are not noted at the output of the various stages.
However, this is not so at the output of the very last stage of a D/A converter. The output node of this last stage is coupled to other circuitry, transmission line, etc, which looks for a continuous analog signal. That is, the output of the last stage of the D/A converter should be a true continuous analog signal. Accordingly, any spurious signals will cause a distortion of the desired analog signal.
The present invention provides for a scheme to reduce distortion at the output of the last stage of a digital-to-analog converter.